A technique for forming a thin film transistor (TFT) by using a thin semiconductor film formed over a substrate having an insulating surface has attracted attention. A thin film transistor is used for a display device typified by a liquid crystal television. A silicon-based semiconductor material is known as a material for a thin semiconductor film applicable to a thin film transistor. As another material, an oxide semiconductor has attracted attention.
As a material of the oxide semiconductor, zinc oxide or a material including zinc oxide as its component is known. Further, a thin film transistor formed using an amorphous oxide (oxide semiconductor) having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Documents 1 to 3).